The present invention relates to nacelle designs for aircraft gas turbine engines and, more particularly, to a nacelle cooling and ventilation system.
As a result of the high temperatures generated within the engine bay of a nacelle housing a gas turbine engine, it is necessary to pass significant amounts of cooling air through the bay to maintain the components within acceptable temperatures and to meet industry standard fire safety criteria. Further, the lubricating oil for the propeller gear box and generator requires a cooling system. Typically, cooling air is supplied through inlets in the nacelle housing into the engine bay and the stream of exhaust air from the engine induces the cooling air from the bay, and mixes it with the exhaust stream. Propeller gear box and generator lubricating oils are cooled in air/oil heat exchangers which receive air from a scoop separate from the engine intake scoop, and release air overboard.
An example of a nacelle cooling system is disclosed in Hotz et al. U.S. Pat. No. 2,548,794. That patent discloses a nacelle in which the twin generators of a twin engine nacelle are connected to receive cooling air from a duct which is separate from the engine inlet duct. An exhaust duct conveys the cooling air from the generators to the exhaust area of the nacelle which is aft of the engine bay so that the exhaust stream from the engines draws air through the generator.
In the Hotz et al. design, the engine bay itself is cooled by entry of air through vents formed in the sides of the nacelle. Air is drawn in through those vents and is passed through the bay and into the inlet air to the engines. Disadvantage with such a design are: (1) any flammable fluid leakage in the engine bay might be carried into the engine inlet with a concurrent fire safety risk, and (2) air drawn in through the side vents decreases the overall efficiency of the engine in developing thrust and reduces the overall aerodynamic efficiency of the nacelle.
Accordingly, there is a need for a nacelle for a gas turbine engine in which both lubricating oil and the engine bay are cooled with external air passed through the bay with minimum reductions in aerodynamic efficiency and in the amount of thrust developed by the engine.